1. Field of the Invention
The present invention relates generally to the field of industrial boilers and furnaces and in particular to new and useful boiler components and attachments having matched materials to improve weld quality and extend the lifespan of the components inside a boiler or furnace.
2. Description of the Related Art
Presently, attachments for boiler components such as superheaters and reheaters which operate in the higher temperature regions of boilers are made of stainless steel materials. Attachments are used to secure the components within the boiler or furnace and include handcuff supports, tie bars and end support lugs. Stainless steel materials are utilized to help resist oxidation of the attachment when exposed to the high operating temperatures.
Ferritic low alloy materials cannot withstand the highly oxidizing environment for long periods of time, and so the use of these materials is limited to locations where temperatures are lower. Commonly, ferritic low alloy materials used in boiler applications are materials such as SA213T22 or SA213T91.
When stainless steel attachments are welded to ferritic low alloy material boiler components, the welds are subject to failure because of the differences in materials. Dissimilar metals do not form as strong welded bonds as when the metals are matched. This dissimilar metal failure mode is common in boiler applications where stainless steel is welded to a ferritic material.
Weld failures cause maintenance problems for the boiler owner or operator, since the weld failures must be repaired on a regular basis to avoid greater damage to unsupported or misaligned boiler components. But, using similar materials for the boiler components and attachments may result in failures of the attachments due to oxidation at elevated temperatures.
Chromizing is a process of producing a chromium diffusion coating on ferrous-base components to improve corrosion resistance, especially at elevated temperatures. Chromizing was developed to produce an integral protective surface coating on components exposed to extreme conditions to enhance their usable life.
Chromizing of parts such as bolts, screws, studs and the like is commonly accomplished using a powder pack cementation technique. The parts are packed into dry powder mixtures of aluminum oxide, chromium, and activator salts within a retort, which is then sealed and heated. During the heating, the temperature of the retort is raised to an elevated level and held for a predetermined amount of time. A chemical reaction takes place during the heating process which causes a surface layer of high chromium content to be diffused into the iron of the ferrous-based parts. The retort and parts are cooled and the parts, now with a layer of chromium coating are removed from the retort.
The pack cementation method has significant drawbacks when used to coat such small, individual parts. The coating thicknesses often vary widely between individual coated parts, or even across the surfaces of larger parts. This is caused by the poor thermal conductivity of the powder pack and the resulting very slow and non-uniform heating rates for the packed parts. Another drawback of this process is that if the parts come into direct contact with each other, the chromium coating diffusion bonds the parts together at these contact points. Further, a large volume of powder waste is generated by the process.
U.S. Pat. No. 4,904,501 discloses an improved method of chromizing the surface of a ferritic boiler component which involves applying aqueous coating compositions containing chromium directly to the surface to be chromized. The aqueous compositions include chromium, alumina, a binder of ammonium alginate or methyl cellulose and a halide activator.
Other processes for coating parts are taught by U.S. Pat. No. 5,135,777, U.S. Pat. No. 5,041,309, U.S. Pat. No. 5,364,659, U.S. Pat. No. 5,492,727 and U.S. Pat. No. 5,589,220.
An improved method of coating small parts, such as bolts, screws and studs, and which could be adapted for use to coat boiler attachments is disclosed in U.S. patent application Ser. No. 08/938,319 filed Sept. 26, 1997, the disclosure of which is incorporated herein by reference.